Better by Nature

Three researchers in WPI’s Civil and Environmental Engineering Department are studying natural materials to adapt elements of their design, or leverage their natural properties, to make materials that are stronger, more durable, and more sustainable.

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Fascinated by biology from an
early age, Nima Rahbar is a
materials scientist who seeks
innovation by studying natural
forms at the nanoscale.

Nature is a master materials designer. Millions of years of relentless adaption have evolved myriad high-performance materials that help living organisms thrive.

Naturally Tough

The blues, greens, and purples of the abalone shell held by assistant professor Nima Rahbar, PhD, shimmer like an iridescent glaze on fragile china. But appearances can be deceiving.

Fascinated by biology from an early age, Rahbar is a materials scientist who seeks innovation by studying natural forms at the nanoscale. “The beauty of nature is that it optimizes for everything, all at the same time,” he says. “If we can learn why these substances are so beautifully tough, then we can apply that knowledge to create better, high-performance materials.”

Toughness is key for Rahbar. He is interested in the ability of a material to resist cracking, or to localize the impact of a small crack so it doesn’t spread and cause the whole system to fail. Glass, he notes, is strong but not tough. A sheet of glass can carry a stable, heavy load. But introduce one crack, and the sheet will shatter.

The beauty of nature is that it optimizes for everything, all at the same time. If we can learn why these substances are so beautifully tough, then we can apply that knowledge to create better, high-performance materials.

Nima Rahbar

In search of natural toughness, Rahbar studies unit cells, which are the smallest repeating forms that give a material its structure. An individual brick, for example, is the unit cell of a masonry well. During his doctoral work at Princeton University, he studied dentoenamel junction, the material that connects enamel to dentin in the core of a tooth. He adapted elements of dentoenamel junction’s design to create a more durable ceramic material for dental crowns.

At WPI since 2012, Rahbar has established the Bioinspired Material Design Lab where he and his students use mechanical tests and computational tools to characterize biologic materials with the aim of developing new materials that leverage beneficial nature properties.

Enter the abalone. The inner lining of its shell (and that of many other mollusks) is called nacre, or mother of pearl, and it’s the source of the shell’s toughness. At the nanoscale, the unit cells of nacre look remarkably like bricks in a wall, with rectangular ceramic plates stacked onto thin layers of protein that functions like mortar. “Nacre is 95 percent ceramic, but the five percent of protein is very important,” Rahbar says. “With the protein, nacre becomes 3,000 times tougher than the ceramic alone.”

Fracture testing of the nacre and computational modeling of the data has drawn Rahbar’s attention to tiny protrusions on the surface of the ceramic plates. “We believe these columns are very important, mechanically,” he says.

Rahbar’s team is in the early stages of creating new ceramic polymers that incorporate the unit cell properties of nacre. One potential applications it the development of lighter bullet-proof vests and body armor. “I believe the best way to solve many of our materials problems is to look at what nature has done, and learn from those lessons,” he says.

Cinnamon Concrete

Aaron Sakulich is exploring ways
to extend the life of modern
concrete by preventing rebar
corrosion.

Cinnamon Concrete

A framed print of the Appian Way hangs on the wall behind the desk of assistant professor Aaron Sakulich, PhD. He rescued it from the attic of Kaven Hall when he came to WPI in 2012 because the iconic Roman roadway reflects Sakulich’s interests in archeology and enduring infrastructure.

“So much of the infrastructure in our country is falling down around our ankles,” Sakulich says. “If we just rebuild with the same materials, we’ll have this trouble again in 50 years.”

The durability of the concrete in Roman bridges, buildings, and aqueducts is legendary, Sakulich says. But the ancient structures have survived nearly 2,000 years primarily because of what they don’t contain: rebar.

Steel reinforcement bars (rebar) are indispensable in modern concrete, as they dramatically extend the material’s strength and design capabilities. But they also embed an Achilles heel. Eventually, rebar corrodes, As rust builds up it exerts pressure on the concrete that leads to cracking. “Once that first crack happens, it’s only a matter of time before you get chunks falling off,” Sakulich said.

Sakulich is exploring ways to extend the life of modern concrete by preventing rebar corrosion. Of particular interest is cinnamaldehyde, which gives cinnamon its aroma and flavor. Sakulich became interested in cinnamaldehyde because it is known to be a natural corrosion inhibitor.

Adding cinnamaldehyde to wet concrete doesn’t work, as it prevents the material from curing properly. So Sakulich engineered a time-release delivery system. Pea-sized bits of porous clay are soaked in cinnamaldehyde, then mixed into the concrete. The clay holds the cinnamaldehyde by capillary action long enough for the concrete to cure.

In the lab, Sakulich’s team fabricates small concrete cylinders with a length of rebar in the center; he calls them concrete lollipops. Some lollipops are made with conventional concrete and others with cinnamaldehyde soaked pellets in the mix. The lollipops are suspended in a salt bath and surrounded by electrodes that deliver a steady charge that drives chlorine ions into the concrete.

So much of the infrastructure in our country is falling down around our ankles. If we just rebuild with the same materials, we’ll have this trouble again in 50 years.

Aaron Sakulich

“The system accelerates weathering and corrosion,” he says. “What would take 25 years in the field, we can simulate in a couple of days.”

The results are encouraging. Conventional concrete lollipops crumble after three to four days, while several cinnamon concrete lollipops have lasted for months. “We have confirmed the cinnamaldehyde is diffusing through the concrete and forming a protective layer on the rebar,” Sakulich says.

Important questions remain—such as whether the cinnamaldehyde layer weakens the bond between the rebar and the concrete, or if the cinnamon oil will leach out of the concrete over time and cause surface problems. “We have more work to do,” Sakulich says. “But the students love the project it because it makes the lab smell like Christmas.”

“The big environmental impact of infrastructure comes from all the detours, traffic jams, and disruption caused by rebuilding roads and bridges,” Sakulich says. “If we can extend even one maintenance cycle by 10 or 15 years, that’s a big positive impact.”

Mingjiang Tao is developing a
geopolymer that will serve as a
sustainable alternative to
Portland cement.

Sustainable Cement

To lay the foundation for a more sustainably built environment, Mingjiang Tao, PhD, studies the chemistry and physics of foundations already in place—the ubiquitous materials used in concrete, asphalt binders, and soil stabilizers.

“Portland cement is the most widely used constructions material in the world,” he says. “It consumes a vast amount of natural resources, it’s caustic, and it’s energy-intensive to make. I believe we can do better.”

Named after stone first quarried on the Isle of Portland in the south of England, Portland cement is the glue that holds most concrete mixtures together. It is made from raw materials found in abundance around the world. “When you consider the energy used for mining, heating, grinding, and finishing the product, the greenhouse gas emissions are one to one,” Tao says. “Making one ton of Portland cement generates one ton of atmospheric carbon dioxide.”

Tao, an associate professor who joined WPI in 2007, applies the tools of engineering and materials science to characterize the essential properties of Portland cement and other cementitious materials. His approach is to meet the global construction needs in a more sustainable way by developing natural or reclaimed materials for use as cement alternatives.

He is currently focusing on a novel geopolymer based on aluminate and silicate that can be synthesized from rice husks or from industrial waste products like rice husks or fly ash generated by coal-burning power plants and municipal trash incinerators. “We wouldn’t need to mine raw materials,” he says. “There is enough industrial waste and natural, renewable sources available today globally to make this geopolymer on a large scale.”

So far, Tao has demonstrated that the geopolymer has similar mechanical properties and can achieve comparable strength to Portland cement. It’s also far more fire resistant and tolerant of acid rain than Portland cement. Tao’s geopolymer can also be used as a more effective soil stabilizer.

Currently, soils are often mixed with Portland cement, lime, or similar materials and compacted to stabilize an area for construction. In many areas of the world, however, the soil is rich in sulfates, which react with calcium present in the stabilizers and expand, gradually damaging the structures built upon it. “The damage done worldwide from soil expansion is probably more than all the damage done by earthquakes, storms, and other natural disasters combined,” Tao says.

Tao’s team has recently completed a proof-of-concept study using the novel geopolymer for concrete mixtures and soil stabilization. The early results are promising, he says, and he is planning the next steps for product development.

As Tao moves his innovations further along the path from the laboratory to the field, he will continue to be attuned to the lessons he can glean from the natural world around him. For like Rahbar and Sakulich, he has learned that sometimes the most advanced, appropriate, and sustainable materials are the ones that have been there all along, just waiting to be rediscovered.

The damage done worldwide from soil expansion is probably more than all the damage done by earthquakes, storms, and other natural disasters combined.